1. Field of the Invention
The subject invention relates to tachometers and to methods and apparatus for manufacturing tachometers of a type having annularly arranged tachometer markings on a rotary tachometer device. By way of example, tachometer devices of the type herein disclosed may be used in the servo control portion of magnetic tape transports, but the utility of the disclosed invention extends beyond that application.
2. Description of the Prior Art
Periodically recurring error signals are a notorious source of malfunction and design and application limitation in servo systems. In particular, a periodic error signal component in a variable input signal of a servo system causes the system to malfunction or at least operate at less than its inherent potential on the one hand and, on the other hand, calls for corrective measures which either render the system expensive or introduce limitations in the full use of the system.
By way of example, periodic error signals in a tachometer signal of a servo control of a rotating drive provoke angular velocity perturbations which affect the performance of the system.
The prior-art effort in this area has been mainly concerned with the source of the periodic disturbance. In particular, most remedial attempts have focused on a mechanical perfection of the parts at the location of generation of the periodic disturbance. Pursuant to this attitude, the typical remedial approach to periodic error signals is in a tachometer signal has culminated in attempts to perfect the tachometer and the equipment coupled thereto and to the associated electromechanical drive. This, in turn, has introduced design and manufacturing complexities, has increased the expense of the resulting equipment, and has subjugated attainable performance to the dictates of mechanical design and feasibility limitations.
Prior-art attempts to solve the problem electronically have not been sufficiently effective as to permit a material reduction of the need for mechanical solutions of the above mentioned type.
For instance, the U.S. Pat. No. 3,312,861, by P. A. Mauch, issued Apr. 4, 1967, proposes the employment of a "mark-read" principle in which a time pulse signal is applied to a recording head for a continuous recording thereof on a rotating tachometer disk. A playback head practically instantaneously reproduces that recorded signal which thereupon is employed to control a servo motor which drives the tachometer disk.
In practice, the utility of that prior-art method is limited to the actual operation of the magnetic tape transport or other equipment which the control servo motor is driving. A reason for this limitation resides in the fact that the prior-art proposal under consideration, as well as any other prior-art technique known to applicants, lacks any facility for a recording of tachometer markings in a continuous circular array. In consequence, even a satisfactory operation of the prior-art proposal under consideration would not lead to a tachometer disk or other rotary tachometer device in which reproducible tachometer markings were arranged in a continuous circular array with no overlapping of markings or similar discontinuities. Accordingly, the prior-art proposal under consideration continuously erases the recorded tachometer markings without ever producing a full circle of markings that could be used over and over in the particular tachometer device.
The closest prior art known to applicants has thus stagnated at a level which does not permit the manufacture of reusable rotary tachometer devices with tachometer markings occupying a continuous circular array. Rather, the prior art has relegated a basically promising principle to a narrow sector limited to each duration of actual use of the equipment.